Why try to get FTP itself to modify the file? Conceptually FTP is supposed to transfer files unmodified. That said I know there are versions of FTP that do all sorts of odd things.
I think the old modem software Procomm would do that or even Kermit. Kermit being a mainframe favorite.
However, it maybe simpler to create a temporary file with the appended control sequence. I seem to remember REXX macros would let you do this on one line (it has been nearly 20 years). Send that.